1. Field of the Invention
This invention relates to a class of actuators capable of configuration to provide high-speed low-torque or to provide low speed high-torque output. The invention includes a method to convert electrical energy directly into mechanical energy utilizing an elastically deformable flexible rotor.
2. Description of Related Art
Conventional actuators of a given power rating, such as electrically or hydraulically powered motors, normally rotate at high speed with low torque. The speed is determined by the electrical excitation frequency and the number of motor poles for an electric motor; or by the flow rate for a hydraulic or pneumatic motor. To obtain low speed and high torque output at rated power, actuators are often coupled with any of several mechanical speed reducers, well known in the mechanical arts, such as chain driven gear sets, belt driven pulley sets and direct gear reduction. Gear reducers may include several stages of speed reduction as in stepped gear reduction or planetary gearing systems. In many industrial applications, high ratio reducers are commonly used with conventional motors running at a relatively high speed, typically 1000 to 5000 rpm or more, to obtain low-speed output rotation with high torque. Otherwise, high power high-torque motors would be used at great penalty of cost, size, and weight.
Some electrically powered motors are commercially available that generate high-torque and operate at low speed by employing a large number of electromagnetic stator poles. However, such motors are usually bulky and expensive. Similarly, low-speed and high-torque hydraulic motors are bulky and heavy with the additional requirement of a separate hydraulic power supply. Applications that are cost sensitive often utilize a high-speed motor coupled to a commercially available gear reduction system such as a multi-ratio gear reducer, a worm gear reducer or a planocentric motion reducer. For many applications it""s desirable that the actuator and the speed reducer are provided with a central hole to pass process cables and lines through motorized joints.
The Harmonic Drive U.S. Pat. No. 3,196,713, is a well known commercial speed reducer which includes a flexible internal shell having externally cut gear teeth that engage a rigid outer shell having internally cut gear teeth. The flexible shell is deformed elliptically by a rotating elliptical cam to engage the outer shell at two diametrically opposed locations. The rotating cam imparts a rotating elastic wave into the flexible shell and causes the shell to rotate about its central axis. The flexible shell is usually coupled to an output shaft that rotates rigidly with it. The difference in the number of teeth between the flexible shell and the outer rigid shell defines the ratio of rotation between the speed of the motor that rotates the cam and the speed of the output shaft. Conventionally, the motor is external to the speed reducer and is coupled mechanically to the elliptical cam. Since the cam is rotated at the high speed of the motor, its inertia negatively impacts the servo controllability of an output load. Alternately, in Ohm, U.S. Pat. No. 4,044,274, the flexible shell is placed within tile air-gap between the motor""s rotor and stator to provide a closely integrated actuator. However, such arrangement increases the width of the air-gap and reduces the power conversion efficiency of the motor.
Another type of motor achieves gear reduction to high torque using a rigid gear shell that progresses within a rigid fixed outer gear having a larger number of gear teeth. A rotating magnetic field, generated by stator poles mounted along the circumference of the outer gear, can induce a rigid style gear shell to roll along the outer gear in an orbital fashion. The low gear shell inertia and the absence of a bulky mechanical element rotating at high speed are features conducive of a desirable low-speed motion control. For example, in Pitchford et al, in U.S. Pat. No. 4,379,976, a rigid orbiting shell progressively engages stator gear teeth and rotor gear teeth along one common line of contact in a planocentric motion. This type of single point loading reduces possible torque output of the motor, promotes vibration, and generates excessive loads on the rotor bearings compared to the present invention. In addition, the motor poles are energized in steps and are not controllable for smooth motion.
Humphreys, U.S. Pat. No. 3,561,006, discloses an electromagnetic actuator having an electromagnetic stator that elliptically deforms a coaxial spline having internal as well as external gear teeth. The coaxial spline progressively engages matching external teeth on an output spline and stator internal teeth, the progressive rotation of the output spline being transmitted to a power output shaft at a rate reduced from the electromagnetic rotation rate. Humphreys employs magnetic shim stock to reduce magnetic reluctance and suggests roughened surfaces instead of gearing for surface engagement. This prior art suffers from the multiplicity of gear engagement surfaces, which is subject to wear, frictional losses and slip with frictional engagement. The, stator, coaxial spline, and output spline elements all serve both torque transmission and magnetic circuit functions. These functions require conflicting material properties of hardness and magnetic reluctance with one usually attained at the detriment of the other. Hence, power conversion efficiency and durability are compromised. The stepping motion of the device is also a serious limitation.
Kondoh et al, in U.S. Pat. No. 5,497,041 (1996) discloses a low-speed motor wherein a rotating magnetic field is formed in a geared outer stator to induce progressive deformation in a geared inner flexible shell containing a series of permanent magnets with alternating polarity. The progressive rotation of the inner flexible shell is transmitted to a power output shaft. In this prior art, the flexible shell is naturally circular and assumed to deform elliptically when the magnetic field is applied. However there is no mechanism to assure such desired elliptical form; the flexible ring could assume the least energy position of single-point contact with the stator and remain circular rather than the desired two-point contact of elliptical deformation which has a higher elastic energy level. The rotor naturally assumes the least energy circular configuration and may jam into a non-rotating vibratory state. In addition, the position of the Kondoh internal gear may become indefinite relative to the position of the rotating magnetic field resulting in compromised precision with this actuator configuration.
The prior art addresses electromagnetic actuators that combine electric motor principles with high gear ratio flexible speed reducers. However, these actuators are impractical for many applications due to the incompatible design considerations involved in combining the functions of electromagnetic permeability and gear engagement in the stator and rotor parts of the motor. Prior art also requires gearing between the rotor and stator elements to avoid slippage in high torque applications. Optimal rotor geometry is not inherent in much of the prior art. These shortcomings are addressed in the present invention.
The unique construction of the present invention overcomes these serious shortcomings and provides other advantages in several ways. In one embodiment, the actuator utilizes the large magnetic attractive forces and friction between the stator and a ferromagnetic rotor flexible shell for the transmission of high torque at low speed, thus avoiding the mechanical complexity and financial cost associated with gearing. Another embodiment includes a series of uniformly polarized permanent magnet segments radially mounted circumferentially to the rotor flexible shell to generate an elliptical rotor shape during electromagnetic interaction with the stator and propagates an elastic wave into the flexible shell. Optimal rotor shape can also be provided by locating an elliptical cam within the rotor flexible shell. The elliptical cam is carried by the sequential flexible shell deformation to rotate synchronously with the electromagnetic field and provides access to high speed mechanical energy. A synchronizing gear element may be provided on the rotor flexible shell to maintain electronic synchronization of rotor position with the electromagnetic field for field commutation and closed loop operation. Energy-conversion efficiency is improved by isolating the gear engagement elements from the magnetic circuit elements, thus allowing the optimum use of materials for each function independent of the other. High precision of motion in servo-controlled low speed drives is obtained by avoiding the need for a high-inertia high-speed rotor or external gear reducer. Low manufacturing cost is realized as fewer mechanical elements are required and high cost load-bearing gearing is eliminated. The Actuator can be built within the confines of a conventional electric motor shell of equal power without the added volume or cost of a speed reducer for low speed output.
The present invention thus provides a low cost, compact actuator that can be designed for optimum performance and manufactured with conventional manufacturing technologies. In all embodiments, the invention allows the rotor to have a relatively large internal axial hole suitable for passing wires and hoses often needed for motorized manufacturing process equipment. Furthermore, for applications such as motorized and remote controlled toys, where low cost is critical regardless of rotor synchronization, this invention provides an ideal actuator with a minimum number of parts for safety and reliability and mass production at a lower cost. The invention provides for an actuator that can be configured to supply low-speed high-torque power output or high-speed low-torque power output or both types of power output from the same actuator.
It is the object of this invention to provide an elastic-wave actuator having a low rotor inertia and an elastically deformable shell that allows: 1) direct electrical energy conversion to mechanical energy with a high torque at low speed; 2) low manufacturing cost; 3) high transfer efficiency between electrical and mechanical energy; 4) minimal lost motion or slipping; 5) compact packaging with minimized rotor inertia; 6) a variety of configurations adaptable to varying application requirements; 7) convenient passage of process lines through a central hole in the actuator; and, 8) an alternate output shaft to provide high speed power output from an elliptical cam.
This invention is an electric energy conversion actuator comprising a stator, a rotor having a flexible shell rotatably supported inside the stator with bearings, and a rigid rotor output flange coupled coaxially to the flexible shell. The stator includes an array of electromagnets arranged along its circumference that are energized to generate a rotating electromagnetic field. Most preferably, the magnetic field attracts and deforms the flexible shell into a substantially elliptical shape to frictionally contact a frictional surface of the stator at two diametrically opposed circumferencial locations. An elastic wave is thus induced into the rotor flexible shell, which progressively rolls along the frictional surface of the stator. Alternately, the flexible shell may be deformed into an elliptical shape by means of a rotatable elliptical cam. Though the elliptical cam is preferred, other cam forms may be used to provide frictional contact at more than two diametrically opposed circumferencial locations. The electromagnets are powered preferably by a multiple phase power supply and synchronized by electronic control means commonly practiced in the art such as direct commutation and sensor directed electronic controls. Pulse width modulated electrical excitation may also be used for precision motion applications. The circumference of the rotor flexible shell differs from the circumference of the frictional surface of the stator by a predetermined amount that causes the flexible shell to rotate at much lower speed than the rotating electromagnetic field. By eliminating the high-speed, high-inertia armature found in most motors, the low-speed rotor output shaft coupled to the flexible shell of this invention supplies high torque with inherently low inertia and high servo-control accuracy. The elliptical cam rotates at the high speed of the rotating magnetic field and provides access to high-speed motion. The flexible shell can be coated with friction promoting material and made of a high electromagnetic permeability metal such as silicon steel to provide high-energy conversion efficiency. Compactness and energy efficiency are also promoted in one embodiment by means of an elastic core of silicon steel sheets preferably laminated and coiled inside or around the flexible shell to maintain adequate magnetic flux path and minimize eddy-current generation. In another embodiment, uniformly polarized permanent magnet segments are mounted circumferentially to the flexible shell to enhance the magnetic flux properties and to maintain the elliptical shape and a holding torque capability when the actuator is not electrically energized. In addition, attractive and repulsive interaction between the permanent magnet segments and the stator electromagnetic poles can induce optimum elliptical shape in the flexible shell.
The rotor flexible shell may be fixedly attached to the rotor output flange, the operative stresses being accommodated by the resilience of the flexible shell. Alternatively, the rotor may have radial splines that interlace with matching radial splines of the rotor output flange to transmit power and accommodate the radial deformations operatively induced in the flexible shell. In one embodiment, the rigid output flange and rotor are mounted using a single moment carrying bearing set at one end of the stator. In another embodiment the rigid output flange extends with a cylindrical shaft to mount on bearing sets at each end of the stator.
In another embodiment, the flexible shell has serrations, or gear teeth, which mesh with matching serrations of equal pitch formed in or near the stator frictional surface. The serration prevent slippage and keep the rotor synchronized with the rotating electromagnetic wave while frictional contact of the flexible shell and stator frictional surface remains the primary means of power transmission. The serrations may have the geometry of conventional gear teeth such as involute, circular, or cycloidal geometry forms and thus provide a definite ratio of speed between the electromagnetic field and the rotor speed. In a preferred embodiment, rotor synchronization can be maintained by a sensor-encoder that provides a feedback-indicating signal of rotor position to an electronic controller system.